The present invention relates to projection screens and more particularly to projection screens having a surface which compensates for projected variations in light intensity levels which are imperceptible to the human observer but are detectable by video reproduction of an image on the screen. The invention also relates to a method for making such projection screens, as well as methods of projecting an image onto such screens.
The projection of slide images on screens is well known. A common problem with such slide projection is that the image projected on the screens tends to be brightest at the center of the screen and dimmest on the edges.
This phenomena is caused by the geometry and optics of conventional slide projectors in that the projector light source projects outward with a beam of light which is in the shape of a sector of a sphere. Since the screen intersects this sphere sector of light with a flat plane, the center of the screen is closer to the light source than the edges. This results in the brightest image portion at the center, with decreasing brightness radially outward therefrom. This phenomena is sometimes termed the "halo" effect.
To the human observer, the halo effect light intensity difference on the screen is imperceptible because the human brain has the capacity to adjust for light intensity variations greater than the differential between the center of the screen and the edges of the screen. Thus, although the human eye can discern the light variations as well as, if not better than, conventional video cameras can, the human brain adjusts for these light variations and the human observer does not perceive these variations. However, reproduction of these projection slide screen images by a video camera detects and records these varying light intensity levels as these video cameras have less capacity to adjust to the varying light levels than the human brain. Therefore, these video cameras will reproduce an image which shows the variation in light intensity to an extent that the video image then includes variations that are Perceptible to the human observer of the video images. Depending on various adjustments, the video image might appear to be of a normal light intensity at the center an appear dark at the edges or might appear to be normal at the edges and excessively bright at the center.
Although the human brain can adjust or accommodate for some of these video recorded light intensity variations, the quality and uniformity of the video signals, as perceived, is lower than would be the case if these slide projector induced light intensity variations were not existent in the video images.
Various methods are presently available which approach the varying light intensity problem by adjusting the light intensity before the light beam reaches the screen. Some methods involve placing a light filter in the path of the light beam between the projector and the screen. These methods require attachments to or adjustments in the video equipment filming the projected image on the screen. Further, these existing processes could easily add few thousand dollars to the cost of a professional quality video recording. A much less expensive approach for reproducing a uniform light intensity video image from a projection screen would be to adjust for the problem on the projection screen itself. Having a projection screen which adjusts for the varying light intensity problem would allow professional quality video recording by a normal video recorder for an indefinite period of time.
A translucent type screen, which adjusts for the varying light intensity problem is shown in U.S. Pat. Nos. 2,242,567 and 2,368,099. In these screens, still or motion pictures are projected on one side of a screen and viewed or photographed on the opposite side of the screen. A transparent base is first coated with a uniform layer of a material which includes microscopic light polarizing crystals. To compensate for the increased intensity at the center of the screen, successive layers of the coating material are applied in a rectangular pattern to produce Progressive increases in thickness of the layer as measured from the edges to the center of the screen. During the operation of spraying the crystal layers, a suitable projector is employed to project a beam of light upon the back surface of the screen. The operator then attempts to spray the successive crystal layers in a rectangular pattern so that a constant intensity over the entire area of the screen is achieved.
There are many problems inherent in the translucent screen discussed above. First, this device only approaches the problem for translucent screens where the image is projected from one side of the screen and viewed from the opposite side of the screen. The successive layers of the coating material which are thicker in the center of the screen and progressively decrease in thickness toward the outer edge of the screen only provide an increased density of quartz crystals in the center of the screen. The increased density in the center of the screen decreases the amount of light which is allowed to pass through the translucent screen. This method of increasing the density of crystals in a translucent screen would not presume a solution for the varying light intensity problem on a screen wherein the projected and viewed image are on the same side.
There are additional problems presented with respect to the methods of constructing the screen. Many coats of the crystal layers are required to achieve an effective density distribution. As stated in the patents, about six to eight coats of the quartz mixture are first evenly applied over the entire surface of the base, then about four or five coats may be applied over a smaller area, next about ten coats may be applied over a still smaller area, next about six coats are applied over the next area, and finally about six coats are applied to the center area of the screen. In all, the preferred embodiment of this invention includes 32-35 coats sprayed on the translucent screen.
Further, during the process of spraying these many coats on the screen, the operator must continually view the light projected through the screen to try and determine if the correct density pattern has been applied. The problems with this procedure are at least twofold. First, as discussed above, often the varied light intensity levels may not be perceived by the human observer. To assure that varying light intensity which is detectable by cameras will be adjusted for, constant video reproduction would be required during the coating process. Second, even if constant video reproduction was provided during the coating process, it would be extremely difficult to achieve an accurate layer by applying rectangular coats, as it is very likely that the light intensity pattern from the projector may have a different geometrical shape, such as spherical or circular to conform to the sphere of light projected from the slide projector.
Another problem encountered in video reproduction of a slide screen image is the so called "halation" effect caused by irregular reflection surfaces at the screen causing a scattering of the reflected light and consequent blurring of the image. The irregular slide screen surface is intentionally designed to accommodate viewing of same by a large audience with different observer locations at different angles with respect to the screen. Without such scattering, the observers located at the edges of the room would not be able to discern a clear image when those in the center of the room in line with the projector light source would see a clear image reflected back. For video recording, there is effectively only one observer at one location, the video camera, and thus the light scattering irregular screen surface features are not needed to improve peripheral viewing as would be the case for normal slide viewing. Thus, one aspect of the invention is to decrease this halation effect to a minimum by appropriate construction of the screen.
A screen has been provided which attempts to decrease light scattering at the reflecting surface due to the irregular nature of the reflecting medium when compared with the wavelength of light. This screen also addresses the problem of reflected ambient light. In this screen 12 layers of the scales of the Scabbard fish are provided on the screen surface. Each layer reflects about 10% of the light falling upon it and 90% of the light passes through to the next layer. The reflected image is made up of 12 successive reflections which all emerge along the same axis. Any remaining ,incident light which is not reflected is absorbed by a black backing material which acts similarly to the antihalation backing layer of film.
Other screens have been constructed which confront reflection problems in images by providing varied textures in the screen. U.S. Pat. No. 2,362,573 shows a screen having lenticular elements in the surface with a reflective material overlay. The lenticular elements intercept all incident light and allow only the light from a projector to be reflected. U.S. Pat. No. 4,089,587 shows a screen having a surface having a combination of random mat texture and a substantially unidirectional striated texture.
The screens discussed above, addressing reflective problems in image reproduction on a screen, all approach the problem by layers having different textures or light reflecting elements such as fish scales or lenticular layers. The present invention adjusts for the halation effect with a smooth one color screen.
Therefore, an object of the present invention is to provide a projection screen which projects an image suitable for reproduction by a video camera.
A further object of the invention is to provide an image projecting screen which compensates for the so called "halo effect" caused by projected variation in light intensity levels imperceptible to the human observer, but detectable and reproduceable by video reproduction of an image on the screen.
Another object of the invention is to provide an image projection screen which compensates for the light scattering so called "halation effect" around the slide image when reproducing the image by a video camera.
Another object of the invention is to provide a relatively inexpensive, efficient and simple method of making an image projector which compensates for the halo effect caused by variation in light intensity levels imperceptible to the human observer but detectable and reproduceable by video reproduction of an image by a video camera.
According to certain preferred embodiments of the invention, an image projector screen is provided having a substrate and a darkened compensating layer on the substrate which compensates for the halo effect caused by projected variations in light intensity levels imperceptible to the human observer, but detectable and recordable by video reproduction of an image on the apparatus.
According to certain preferred embodiments of the invention, an image projection screen is provided with a substrate having a darkened absorbing layer applied to the substrate to minimize the halation effect. This darkened absorbing compensating layer device absorbs interfering light and reduces light scattering otherwise detectable by video reproduction of the image and, therefore, results in a clearer, sharper video image.
Certain preferred embodiments of the invention include both the compensating layer and the absorbing layer. Other contemplated embodiments of the invention include only one of these layers.
Certain preferred methods of manufacturing a slide projector screen according to the invention, include applying a darkened compensating surface to a substrate which compensates for the halo effect caused by projected variations in light intensity levels having an intensity imperceptible to the human observer but detectable by video. An image is then projected on this apparatus. In especially preferred methods of constructing the screen, a grey oil base surface is silk-screened on a substrate. A gelatinous photographic emulsion layer having a light sensitivity similar to photographic film is then applied. The photographic emulsion layer is then exposed to light by using a positive film image of a predetermined average projection pattern of an image projector. The photographic emulsion layer, when developed and fixed, forms a very accurate reverse image of the average projection pattern projected thereon and thereby compensates for the halo effect in a single manner.
In especially preferred embodiments, the positive film image which is used to expose the emulsion layer on the slide projection screen is made by the sequential steps of:
i. setting up a slide projection screen and slide projector with a still camera mounted on or adjacent the slide projector; PA1 ii. using a test slide to focus the slide image on the screen; PA1 iii. removing the test slide and leaving the slide projector light on; PA1 iv. using the still camera to photograph the illuminated screen; and PA1 v. using the negative of step iv. to expose another film sheet to make a master positive film sheet.
The positive sheet film image is then used to expose the emulsion layer on the projection screen being manufactured.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.